This invention relates generally to obtaining residual voltage values in an ungrounded network which includes a xe2x80x9cbroken deltaxe2x80x9d voltage transformer (VT) connection, and more specifically concerns such a system which uses phase voltages obtained from the VT connection to determine the residual voltage values.
Voltage transformers (VTs) for a three-phase power line can be wired into different primary/secondary configurations. The input (primary) side of the voltage transformers receives the three phases of the power line voltage signal, while the output (secondary) side produces reduced voltage values which are then applied to a protective relay or more particularly another set of voltage transformers in a protective relay, which reduce the voltage values from the power system VTs to levels which can be used by the processing circuitry in the protective relay.
One particular power system VT configuration is shown in FIG. 1 and is known as a xe2x80x9cbroken deltaxe2x80x9d VT. In a broken delta VT, the non-polarity end of the A-phase secondary winding 11 is connected directly to the polarity end of the secondary B-phase winding 12, while the non-polarity end of the B-phase secondary winding 12 is directly connected to the polarity end of the C-phase secondary winding 13. The polarity end of the A-phase winding and the non-polarity end of the C-phase winding are used directly from output lines 14 and 16 to produce a residual voltage 3 V0 between them. This directly obtained 3 V0 value is used to monitor and control the power on the power line. The 3 V0 value is equal to the sum of the individual phase currents VA+VB+VC.
The 3 V0 output from the broken delta VT secondary connection, however, has a relatively high voltage value, i.e. approximately 360 volts nominal voltage. This level of voltage requires the VTs in the protective relay to have a higher voltage rating than normal, which increases expense of the system. Also, the protective relay operating with a 3 V0 value from a broken delta VT connection cannot be used to check for blown potential fuses in the power system voltage transformers, since the 3 V0 value from the broken delta VT remains the same, i.e. zero volts, both before and after a fuse blows. Still further, the broken delta VT connection with 3 V0 values is typically limited to ground overcurrent protection and cannot be used for other protection schemes, such as phase-to-phase and three phase protection.
Hence, it would be desirable to be able to use a broken delta VT connection system with lower rated VTs in the protective relay and to be able to use such a system with other protection schemes besides ground overcurrent. Still further, it would be desirable to have a protection arrangement which could be responsive not only to a broken delta VT connection, but also other types of VT secondary arrangements, such as open delta VT connections.
Accordingly, the present invention is a system for obtaining phase-ground voltages from a broken delta voltage transformer arrangement used in an electric power system, comprising: a voltage transformer arrangement in a protective relay responsive to a power system voltage transformer arrangement having a broken delta secondary connection, wherein the primary windings of the relay voltage transformer arrangement are in a broken delta connection, matching the secondary connection of the power system voltage transformer arrangement, wherein the relay voltage transformer arrangement includes three separate secondary windings to produce phase voltage VA, VB and VC.